Fluxless bumping process

ABSTRACT

A process including providing a semiconductor device including a bond pad, and an under bump metallurgy overlying the bond pad. Forming a solder structure over the under bump metallurgy, and wherein the solder structure includes an outer layer including tin oxide. Producing a plasma from at least one of CF 4  and SF 6 , and exposing the solder structure to the plasma. Heating the solder structure and cooling the same to provide a solder bump on the semiconductor device.

FIELD OF THE INVENTION

This invention relates to a method of making an electrically conductivebump on a substrate, and more particularly to a method of making anelectrically conductive bump on a substrate without using a traditionalflux.

BACKGROUND OF THE INVENTION

It is common in the semiconductor industry to make an electricallyconductive bump on a substrate, and in particular to make a solder bumpon a semiconductor device. Solder bumps are often formed on anintegrated circuit chip to make a flip chip. A flip chip microelectronicassembly includes a direct electrical connection of face down (that is,“flipped”) electronic components onto substrates, such as ceramicsubstrates, circuit boards, or carriers using conductive bump bond padsof the chip. Flip chip technology is quickly replacing older wirebonding technology that uses face up chips with a wire connected to eachpad on the chip.

The flip chip components used in flip chip microelectronic assembliesare predominantly semiconductor devices, however, components such aspassive filters, detector arrays, and MEM devices are also being used inflip chip form. Flip chips are also known as “direct chip attach”because the chip is directly attached to the substrate, board, orcarrier by the conductive bumps.

The use a flip chip packaging has dramatically grown as a result of theflip chip's advantages in size, performance, flexibility, reliability,and cost over other packaging methods and from the widening availabilityof flip chip materials, equipment and services. In some cases, theelimination of old technology packages and bond wires may reduce thesubstrate or board area needed to secure the device by up to 25 percent,and may require far less height. Further, the weight of the flip chipcan be less than 5 percent of the old technology package devices.

Flip chips are advantageous because of their high-speed electricalperformance when compared to other assembly methods. Eliminating bondwires reduces the delay in inductance and capacitance of the connection,and substantially shortens the current path resulting in a high speedoff-chip interconnection.

Flip chips also provide the greatest input/output connectionflexibility. Wire bond connections are generally limited to theperimeter of the chip or die, driving the die sizes up as a number ofconnections have increased over the years. Flip chip connections can usethe whole area of the die, accommodating many more connections on asmaller die. Further, flip chips can be stacked in 3-D geometries overother flip chips or other components.

Flip chips also provided the most rugged mechanical interconnection.Flip chips when underfilled with an adhesive such as an epoxy, canwithstand the most rugged durability testing. In addition to providingthe most rugged mechanical interconnection, flip chips can be the lowestcost interconnection for high-volume automated production.

The bumps of the flip chip assembly serve several functions. The bumpsprovided an electrical conductive path from the chip (or die) to thesubstrate on which the chip is mounted. A thermally conductive path isalso provided by the bumps to carry heat from the chip to the substrate.The bumps also provided part of the mechanical mounting of the chip tothe substrate. A spacer is provided by the bumps that preventselectrical contact between the chip and the substrate connectors.Finally, the bumps act as a short lead to relieve mechanical strainbetween the chip and the substrate.

Flip chips are typically made by a process including placing solderbumps on a silicon wafer. The solder bump flip chip processing typicallyincludes four sequential steps: 1) preparing the wafer for solderbumping; 2) forming or placing the solder bumps on the wafer; 3)attaching the solder bumped die to a board, substrate or carrier; and 4)completing the assembly with an adhesive underfill.

The first step in a typical solder bumping process involves preparingthe semiconductor wafer bumping sites on bond pads of the individualintegrated circuits defined in the semiconductor wafer. The preparationmay include cleaning, removing insulating oxides, and preparing a padmetallurgy that will protect the integrated circuits while making goodmechanical and electrical contact with the solder bump. Accordingly,protective metallurgy layers may be provided over the bond pad. Balllimiting metallurgy (BLM) or under bump metallurgy (UBM) generallyconsists of successive layers of metal. The “adhesion” layer must adherewell to both the bond pad metal and the surrounding passivation, providea strong, low-stress mechanical and electrical connection. The“diffusion barrier” layer prevents the diffusion of solder into theunderlying material. The “solder wettable” layer provides a wettablesurface for the molten solder during the solder bumping process, forgood bonding of the solder to the underlying metal.

A variety of UBM structures are known to those skilled in the art thataccomplish the above functions and have one, two, three or more layersdepending on whether the bump is gold, copper, aluminum, solder ornickel based. For gold based bumps, known UBM structure include layersof Cr—Cu, Ti—Pd, Ti—W, or Ti—Pt. For copper based bumps, known UBMstructures include layers of Cr—Cu, or Al—Ni. For aluminum based bumps,known UBM structure include layers of Ti or Cr. For solder based bumps,known UBM structures include layers of Cr—Cu—Au, Ni—Cu, Ti—Cu, TiW—Cu,Ni—Au, or Al—NiV—Cu. For nickel based bumps, known UBM structure includelayers of nickel. The UBM layers may be deposited by electroplating,evaporation, printing, electroless plating, and/or sputtering. It isalso known to deposit one or more seed layers over the UBM structureprior to depositing the electrically conductive material (such assolder) that forms the bump.

In fabricating a flip-chip bond structure, the fabrication processrequires a tight control of interface processes and manufacturingparameters in order to meet very small dimensional tolerances. Varioustechniques may be utilized to fabricate a UBM structure and to depositthe solder bump. A few widely used methods of depositing bumps includeevaporation, electroplating, electroless plating and screen-printing.Kung et al, U.S. Pat. No. 6,179,200 provides a description of these morewidely used methods of depositing bumps as follows.

The formation of solder bumps can be carried out by an evaporationmethod of Pb and Sn through a mask for producing the desired solderbumps. When a metal mask is used, UBM metals and solder materials can beevaporated through designated openings in the metal mask and bedeposited as an array of pads onto the chip surface.

In one prior art evaporation method, a wafer is first passivated with aninsulating layer such as SiO₂, via holes are then etched through thewafer passivation layer to provide a communication path between the chipand the outside circuit. After a molybdenum mask is aligned on thewafer, a direct current sputtering cleans the via openings formed in thepassivation layer and removes undesirable oxides. A cleaned via openingassures low contact resistance and good adhesion to the SiO₂. A chromiumlayer is evaporated through a metal mask to form an array of round metalpads each covering an individual via to provide adhesion to thepassivation layer and to form a solder reaction barrier to the aluminumpad underneath. A second layer of chromium/copper is then co-evaporatedto provide resistance to multiple reflows. This is followed by a finalUBM layer of pure copper which forms the solderable metallurgy. A thinlayer of gold may optionally be evaporated to provide an oxidationprotection layer. These metal-layered pads define the solder wettableregions on the chips, which are commonly referred to as the balllimiting metallurgy (BLM) or under bump metallurgy (UBM). After thecompletion of UBM, solder evaporation occurs through a metal mask, whichhas a hole diameter slightly greater than the UBM mask-hole diameter.This provides the necessary volume for forming a subsequent solder ball.A solder reflow process is performed at a temperature of about 350° C.to melt and homogenize the evaporated metal pad and to impart atruncated spherical shape to the solder bump. The evaporation method,even though well established and has been practiced for a long time inthe industry, is a slow process and thus can not be run at a highthroughput rate.

A second method for forming solder bumps is the electroplating method.In an electroplating process, UBM layers are first deposited, followedby the deposition of a photoresist layer, the patterning of thephotoresist layer, and then the electro-deposition of a solder materialinto the photoresist openings. After the electro-deposition process iscompleted, the photoresist layer can be removed and the UBM layers canbe etched by using the plated solder bumps as a mask. The solder bumpsare then reflowed in a furnace reflow process. Thephotolithography/electroplating technique is a simpler technique thanevaporation and is less expensive because only a single maskingoperation is required. However, electroplating requires the depositionof a thick and uniform solder over the entire wafer area and etchingmetal layers on the wafer without damaging the plated solder layer. Thetechnique of electroless plating may also be used to form the UBMstructure.

Another solder bump formation technique that is capable ofsolder-bumping a variety of substrates is a solder paste screeningmethod. The screen printing technique can be used to cover the entirearea of an 8-inch wafer. In this method, a wafer surface covered by apassivation layer with bond pads exposed is first provided. UBM layersare then deposited on top of the bond pads and the passivation layer. Aphotoresist layer is deposited over the UBM. The portions of the UBM areetched followed by stripping off the photoresist layer. A stencil isthen aligned on the wafer and solder paste is squeegeed through thestencil to fill the openings on top of the bond pads and the UBM layers.After the stencil is removed, the solder bumps may be reflowed in afurnace to form solder balls.

One drawback of the solder paste screen printing process is that, withthe recent trend in the miniaturization of device dimensions and thereduction in bump to bump spacing (or pitch), the prior art solder pastescreening techniques become impractical. For instance, one of theproblems in applying solder paste screening technique to modern ICdevices is the paste composition itself. A paste in generally composedof a flux and solder alloy particles. The consistency and uniformity ofthe solder paste composition becomes more difficult to control with adecreasing solder bump volume. A possible solution for this problem isthe utilization of solder paste that contains extremely small anduniform solder particles. However, this can only be achieved at a veryhigh cost penalty. Another problem is using the solder paste screeningtechnique in modern high-density devices is the reduced pitch betweenbumps. Since there is a large reduction in volume from a paste to theresulting solder bump, the screen holes must be significantly larger indiameter than the final bumps. It is therefore generally desirable toform solder bumps that are reflown into solder balls with a largerheight and a larger pitch between the balls.

Several other methods are known to those skilled in the art forproducing solder bumps on a semiconductor device. One such method iscalled the solder jet printing method. The solder jet printer method isbased upon piezoelectric demand mode ink jet printing technology and iscapable of producing and placing molten solder droplets 25–125micrometers in diameter at rates of up to 2000 per second. In demandmode ink jet printing systems, a volumetric change in the fluid isinduced either by the displacement of piezoelectric material that iscoupled to the fluid or by the formation of the vapor bubble in the inkcaused by heating a resistive element. The volumetric change causespressure transience to occur in the fluid, and these are directed so asto produce a drop that issues from an orifice. A droplet is created onlywhen it is desired in demand mode systems. Demand mode ink jet printingproduces droplets that are approximately equal to the orifice diameterof the droplet generator.

Another method for producing solder bumps is known as the micro-punchingmethod. In the micro-punching method, solder tape is supplied from aspool and rolled up by a motor driven spool. A micro-punch is driven byan electric actuator and a displacement enlarging mechanism. Amicro-punch and die set blanks a thin solder tape and forms a smallcylindrical piece. A solder flux may be formed over the entiresemiconductor wafer to be bumped and the solder pieces may be punchedand placed directly onto the wafer.

One of the most cost-effective packaging techniques is known as directchip attach wherein a solder bumped flip chip is directly attached to aprinted circuit board. However, due to the thermal expansion mismatchbetween the silicon chip and the printed circuit board (made from anepoxy or fiberglass material), an underfill encapsulant is usuallyneeded for solder joint reliability. Due to the underfill operation, themanufacturing costs is increased in the manufacturing throughput isoften substantially reduced. Further, reworking an underfill flip chipon a printed circuit board is practically impossible.

Although electrically conductive bumps, such as solder bumps, are morefrequently used with flip chips, electrically conductive bumps are oftenapplied to other semiconductor devices, printed circuit boards, andceramic substrates. Typically the forming of solder bumps isaccomplished using a flux, which in the semiconductor industry usuallyis an organic oxide. FIGS. 1A–F illustrate the steps of a typical priorart process of forming a solder bump on a substrate. In FIG. 1A asemiconductor device 10 is provided including a silicon based substrate12 with a plurality of alternating metallization and dielectric layers.A bond pad 14 is provided over the silicon based substrate 12. Apassivation layer 16 is provided over the silicon based substrate 12 andincludes an opening down to the bond pad 14. An under bump metallurgy 18is provided over the passivation layer 16 and into the opening formed apassivation layer 16 and is connected to the bond pad 14. In FIG. 1B asacrificial layer 20 is provided which typically is a photoresist layerthat has been deposited, patterned and developed to provide an opening22 therein overlying the bond pad 14. In FIG. 1C, solder 24 may bedeposited into the opening 22 formed in the sacrificial layer 20 andonto the under bump metallurgy 18. The solder 24 may be deposited by anyof a variety of methods including electroplating or screen printing. Asshown in FIG. 1D the sacrificial layer 20 is then removed to provide asolder structure 26 on top of the under bump metallurgy 18. It istypical for the solder structure 26 to form an oxide layer 32, moreparticularly thin oxide (SnO_(x)) upon exposure to the air for evenshort periods of time. Excess under bump metallurgy 18 is removedtypically by etching using the solder structure 26 as a mask, and thenas shown in FIG. 1E, a solder flux 28 is deposited over the solderstructure 26. The flux is particularly needed if the solder iselectroplated onto the substrate because an electroplated solderstructure would not have flux included in the composition of the solder.The purpose of the solder flux is to reduce the oxide on the surface tobe heated or material (solder) to be reflown. The reduction of theoxides allows the solder to flow at a lower temperature. As shown inFIG. 1F, the solder structure 26 with the flux 28 is heated to reflowthe solder structure 26 and then cooled to form a solder bump 30 on thesilicon based substrate 12.

However, the use of a flux agent requires additional applications steps,additional process equipment tools and often follow-up cleanup steps toremove flux residue. Flux residue also has associated with it a highrisk of reliability due to corrosion caused by the flux residue. Thepresent invention provides alternatives to and advantages over the priorart.

SUMMARY OF THE INVENTION

One embodiment of the invention includes a process including providing asubstrate having an electrically conductive structure thereon, theelectrically conductive structure including an outer surface including acompound including an oxide group. Converting the compound including theoxide group to a compound including an oxy-halide group thereby reducingthe melting temperature of the outer surface. Heating the electricallyconductive structure including the compound including the oxy-halidegroup and cooling the same to form an electrically conductive bump onthe substrate.

In another embodiment of the invention the converting of the compoundincluding the oxide group to the compound including an oxy-halide groupcomprises exposing the electrically conductive material to a plasmacontaining halide ions.

In another embodiment of the invention the halide ions comprise F ions.

Another embodiment of the invention further includes producing a plasmafrom a gas including at least one of carbon tetrafluoride and sulfurtetrafluoride and wherein converting the compound including the oxidegroup to a compound including the oxy-halide group comprises exposingthe electrically conductive material to the plasma.

Another embodiment of the invention includes a process includingproviding a substrate having a solder structure thereon, the solderstructure including an outer surface including tin oxide. Converting thetin oxide to tin oxy-halide thereby reducing the melting temperature ofthe outer surface. Heating the solder structure including the tinoxy-halide and cooling the same to form a solder bump on the substrate.

In another embodiment of the invention the converting of the tin oxideto tin oxy-halide comprises exposing the solder to a plasma includinghalide ions.

In another embodiment of the invention the halide ions comprise F ions.

Another embodiment of the invention includes providing a semiconductordevice including a bond pad, and an under bump metallurgy overlying thebond pad. Forming a solder structure over the under bump metallurgy, andwherein the solder structure includes an outer layer including tinoxide. Converting the tin oxide to tin oxy-halide to reduce the meltingtemperature of the outer layer. Heating the solder structure and coolingthe same to provide a solder bump on the semiconductor device.

Another embodiment of the invention includes providing a semiconductordevice including a bond pad, and an under bump metallurgy overlying thebond pad. Forming a solder structure over the under bump metallurgy, andwherein the solder structure includes an outer layer including tinoxide. Producing a plasma from at least one of CF₄ and SF₆, and exposingthe solder structure to the plasma. Heating the solder structure andcooling the same to provide a solder bump on the semiconductor device.

In another embodiment of the invention the forming of the solderstructure comprises forming a sacrificial layer over the semiconductordevice, and the sacrificial layer having an opening therein overlyingthe under bump metallurgy, depositing solder into the opening, andremoving the sacrificial layer to provide the solder structure.

In another embodiment of the invention the depositing of the solder intothe opening comprises electroplating solder into the opening.

In another embodiment of the invention the depositing of the solder intothe opening comprises screen printing a fluxless solder composition intothe opening.

Another embodiment of the invention includes providing a semiconductordevice including a solder structure thereon, the solder structureincluding an outer surface including tin oxide. Exposing the outersurface to a plasma including F ions. Reflowing the solder structure toform a solder bump on the semiconductor device.

These and other objects, features and advantages of the presentinvention will become apparent from the following brief description ofthe drawings, detailed description of the preferred embodiments, andappended claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates the forming of an under bump metallurgy over asemiconductor substrate, bond pad, and passivation layer overlying thesubstrate that includes an opening down to the bond pad according to aprior art process;

FIG. 1B illustrates the forming of a sacrificial layer having an openingtherein down to the under bump metallurgy of the structure in FIG. 1Aaccording to a prior art process;

FIG. 1C illustrates the depositing of solder into the opening in thesacrificial layer and onto the under bump metallurgy according to aprior art process;

FIG. 1D illustrates the removing of the sacrificial layer to provide asolder structure on the under bump metallurgy according to a prior artprocess;

FIG. 1E illustrates the depositing of a flux over the solder structureaccording to a prior art process;

FIG. 1F illustrates the heating of the solder structure and the flux andcooling the same to form solder bump on a semiconductor device accordingto a prior art process;

FIG. 2A illustrates the forming of an under bump metallurgy over asemiconductor substrate, bond pad, and passivation layer overlying thesubstrate that includes an opening down to the bond pad according to thepresent invention;

FIG. 2B illustrates the forming of a sacrificial layer having an openingtherein down to the under bump metallurgy of the structure in FIG. 1Aaccording to the present invention;

FIG. 2C illustrates the depositing of an electrically conductivematerial into the opening in the sacrificial layer and over the underbump metallurgy according to the present invention;

FIG. 2D illustrates the removing of the sacrificial layer to provide anelectrically conductive structure (including an oxide layer) over theunder bump metallurgy according to the present invention;

FIG. 2E illustrates the converting of the oxide on the electricallyconductive structure to an oxy-halide according to the presentinvention; and

FIG. 2F illustrates heating of the electrically conductive structure andcooling the same to form an electrically conductive bump on asemiconductor device according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2A–F illustrate the forming of an electrically conductive bump ona substrate according to the present invention. In FIG. 1A asemiconductor device 10 is provided including a silicon based substrate12 (such as a semiconductor wafer) with a plurality of alternatingmetallization and dielectric layers (not shown). A bond pad 14 isprovided over the silicon based substrate 12. A passivation layer 16 isprovided over the silicon based substrate 12 and includes an openingdown to the bond pad 14. An under bump metallurgy 18 is provided overthe passivation layer 16 and into the opening formed in the passivationlayer 16 and is connected to the bond pad 14. In FIG. 2B a sacrificiallayer 20 is provided which typically is a photoresist layer that hasbeen deposited, patterned and developed to provide an opening 22 thereinoverlying the bond pad 14. In FIG. 2C, an electrically conductivematerial 24 such as a material including solder, gold, copper, aluminumand/or nickel (or any other bumping material known to those skilled inthe art) may be deposited into the opening 22 formed in the sacrificiallayer 20 and over the under bump metallurgy 18. Preferably theelectrically conductive material 24 includes solder in any compositionknown to those skilled in the art including 63 weight percent Sn and 37weight percent Pb. The electrically conductive material 24 may bedeposited by any of a variety of methods including electroplating,screen printing, evaporation, electroless plating, jet printing,micro-punching or any other method known to those skilled in the art. Asshown in FIG. 2D the sacrificial layer 20 is then removed to provide anelectrically conductive structure 26 on top of the under bump metallurgy18. It is typical for the electrically conductive structure 26 to forman oxide layer 32, more specifically tin oxide (SnO_(x)) in the case ofsolder, upon exposure to the air for even short periods of time. Excessunder bump metallurgy is removed typically by etching using theelectrically conductive structure 26 as a mask, and then as shown inFIG. 2E, the oxide layer is converted to an oxy-halide which reduces thereflow temperature of the electrically conductive structure 26.Preferably this is accomplished by providing a plasma of halide ions,and more preferably a plasma of F ions, and exposing the electricallyconductive structure (preferably solder) to the plasma of halide ions toconvert the oxides to oxy-halides. The plasma of halide ions can beprovided by exciting gas molecules containing halide atoms, such as CF₄or SF₆. In a preferred embodiment the electrically conductive structure26 includes solder having tin oxide which may be in the form of SnO_(x).Exposing the SnO_(x) to a plasma including F ions converts the SnO_(x)to SnO_(x)F_(y). The reduction or conversion of the oxides tooxy-halides allows the electrically conductive material or solder toflow at a lower temperature. As shown in FIG. 2F, the electricallyconductive structure 26 with the outer surface including oxy-halides isheated to reflow the electrically conductive structure 26 and thencooled to form an electrically conductive bump 30 (preferably solderbump) on the silicon based substrate 12. The invention is particularlyapplicable to electrically conductive structures 26 that have beendeposited over formed from a composition that is substantially free offlux agents or has no flux agents. The invention is a fluxless method offorming bumps on a substrate.

1. A process for forming a solder bump to avoid flux residuecontamination and reduce a reflow temperature comprising: providing asubstrate having an electrically conductive structure thereon, theelectrically conductive structure including an outer surface including acompound including an oxide group; and converting the compound includingan oxide group to a compound including an oxy-halide group therebyreducing the melting temperature of the outer surface comprisingexposing the electrically conductive structure to a plasma comprising Fion, and thereafter heating the electrically conductive structureincluding the compound including the oxy-halide group and cooling thesame to form an electrically conductive bump on the substrate.
 2. Aprocess as set forth in claim 1 wherein the electrically conductivestructure comprises solder.
 3. A process as set forth in claim 1 whereinthe plasma is produced from a gas including at least one of CF₄ and SF₆.4. A process for forming a solder bump to avoid flux residuecontamination and reduce a reflow temperature comprising: providing asubstrate having a solder structure thereon, the solder structureincluding an outer surface including tin oxide; converting the tin oxideto tin oxy-halide thereby reducing the melting temperature of the outersurface comprising exposing the tin oxide to a plasma comprising F ionsand thereafter heating the solder structure including tin oxy-halide andcooling the same to form an solder bump on the substrate.
 5. A processas set forth in claim 4 wherein the plasma is produced from a gasincluding at least one of CF₄ and SF₆.
 6. A process as set forth inclaim 5 wherein the substrate comprises a semiconductor device.
 7. Aprocess as set forth in claim 6 wherein the semiconductor deviceincludes a silicon base substrate and a plurality of alternatingmetallization and dielectric layers.
 8. A method as set forth in claim 4further comprising an under bump metallurgy underlying the solderstructure.
 9. A process for forming a solder bump to avoid flux residuecontamination and reduce a reflow temperature comprising: providing asemiconductor device including a solder structure thereon, the solderstructure including an outer surface including tin oxide; exposing theouter surface to a plasma including F ions to convert the outer surfaceinto an oxy-halide surface and thereafter reflowing the solder structureto form a solder bump on the semiconductor device.
 10. A process forforming a solder bump to avoid flux residue contamination and reduce areflow temperature comprising: providing a semiconductor devicecomprising a solder structure for forming a solder bump, said solderstructure on an underbump metallurgy (UBM); then plasma treating thesolder structure to convert the solder structure into an oxy-halidesurface, said plasma treatment comprising a plasma source gas selectedfrom the group consisting of CF₄ and SF₆; then reflowing the solderstructure to form the solder bump.
 11. The method of claim 10 whereinsaid UBM is on a bonding pad.
 12. The method of claim 10 wherein thesolder structure is formed by screen printing a solder composition intoan opening formed in a sacrificial layer on the semiconductor device.13. The method of claim 10 wherein the solder structure is formed by anelectroplating process.